Answer:
To find the mass of water we first measure the mass of an empty cylinder, m1, and then add the water and measure the new mass, m2, since we need only the mass of water we remove the mass of the beaker let me give you an example. Suppose your dad gave you 6 dollars and I gave you 9 dollars (I am generous ik :)) so now you would have 15 dollars but If anyone asks you how much I give what would you do? You would subtract the total from your dad's given money because after I gave you the money increase to find that increase, we minus the masses.
m of water= 147-115= 32g
Think like your dad gave you 115 grams and I gave you 32 so in total you have 147 grams. I hope this helped.